1. Field of the Invention
The present invention generally relates to a circuit for driving a light emitting diode (LED), and more particularly, to a circuit and a method for driving light emitting diode strings (LED strings).
2. Description of Related Art
Due to advantages of electricity-saving and a fast switching speed, the applications of LEDs are much broader today than before where they were used as status lights for electronic devices in the earliest application, later advanced to serve as a backlight of an LCD and further advanced to serve as an electronic lighting and public display, such as vehicle light, traffic light, moving message board, large-scale video wall and even lighting inside a projector. After a high-end handset adopts LEDs as the backlight source thereof, LEDs have entered another new application field. The most potential application for LEDs is the market of 7-40 inch flat panel display. Once LEDs become a backlight source of plat panel display, a significant increase in production value on the market is expected.
The LEDs applied in the above-mentioned various products include a plurality of LEDs connected in series and parallel. Since the conducting voltage of each LED on a driving circuit employing a plurality of LEDs connected in series and parallel may differ from each other, the conventional scheme to prevent the conducting voltage of an LED string from being excessively high to fail lighting is to preset the output voltage of the driving circuit at a higher level.
FIG. 1 is a diagram of a conventional driving circuit for a plurality of LEDs. It can be seen from FIG. 1, the output voltage VOUT of the circuit is determined by feedback resistors R1 and R2, wherein the output voltage VOUT needs to be at least greater than the highest level among the conducting voltages required by each of LED strings SL1-SLN plus the lowest voltage required by the current source in the driving circuit so that the output voltage is sufficient to make all the LED strings SL1-SLN emit light (conducted).
Although the above-mentioned circuit of FIG. 1 can employ a voltage source (with an output voltage VOUT) to drive each of the LED strings SL1-SLN; however, each LED in the LED strings SL1-SLN would produce different conducting voltage due to a process nonconformance thereof. Therefore, in order to drive all the LEDs, the prior art presets the output voltage OUT at a higher level to prevent any LED string with a greater conducting voltage from failing to light. Nevertheless, the scheme of presetting the output voltage VOUT at a higher level would make the current source 101 applied by a greater voltage drop, which would lower the energy conversion efficiency and consume more power.